Legal mandates in some countries require every operator of a cellular communications system to provide means for estimating and reporting the position of any mobile terminal operating within its network in the case of an emergency. Positioning methods applied to mobile terminals are also being used for location-based services in which applications provide information based on the estimated position of a terminal. Several positioning methods and systems have been developed in attempts to satisfy the various requirements, including ones which use SPS signals and others which use the signals radiated within the communications network itself, flowing in either direction between the mobile terminal and the network base transceiver stations. Satellite positioning systems include the Global Positioning system (GPS), GLONASS, and Galileo. These all operate in a similar way. Satellites in near-Earth orbit emit ranging signals which can be received and decoded to obtain estimates of the distances between the receiving terminal and each satellite. Four or more such estimates are usually sufficient to provide a three-dimensional position fix together with time measured with respect to the SPS time base.
In the case of GPS, the signals available for civilian use are radiated on 1575.42 MHz (known as L1) and on 1227.60 MHz (known as L2). The L1 carrier signals are bi-phase modulated with a ranging/spreading code, different for each satellite, at a chipping rate of 1.023 MHz. The code is a pre-defined member of the Gold family as specified in a US Government Publication—an Interface Standard IS-GPS 200. The civilian code is called the CA code and it has a length of 1023 chips, repeating every 1 ms.
The code epoch, i.e. its starting point, is defined in IS-GPS 200 and corresponds to the occurrence of a certain state of the CA code generator. The code epoch is aligned with a high-precision (atomic) clock carried on board the satellite. A number of terrestrial control centres, known collectively as the ground-based control segment, continuously monitor and steer the satellite clocks towards alignment with the GPS time-base, known as GPS time. This is held in a clock located at the United States Naval Observatory in Washington D.C. and is a continuous time scale which is itself aligned, typically within a few micro-seconds (apart from leap seconds) with Universal Coordinated Time (UTC). Each satellite clock may have a time error with respect to GPS time of up to 976.6 ms. The difference between GPS time and that held in the satellite clock is broadcast to users by the satellite using three parameters known as the clock correction parameters.
The ground-based control segment also measures and estimates the location and orbit of each satellite. Knowledge of the track actually followed by each satellite, its current position, and estimates of the effects of the gravity fields of the Sun, Moon and Earth, the effects of the Solar wind and some other smaller forces, are used to predict the future orbits of each satellite which is then broadcast to all users. The orbital parameters are known collectively as the satellite ephemeris, and they can usually be used to extrapolate over periods of a few hours. A lower-precision version of the orbit, called the satellite almanac, with a life of several months or more, is also broadcast.
A GPS terminal containing a radio receiver, ancillary hardware, and one or more processors running software programs, is often collectively known as a ‘GPS receiver’. Hereinafter, we understand the term ‘receiver’ in this wider sense to include all the parts necessary to carry out the functions of receiving, decoding signals, and processing data. A GPS receiver, therefore, receives the satellite ranging signals, the satellite clock correction parameters and the satellite ephemeris and almanac messages, for one or a plurality of satellites, and is able to calculate the position and velocity of the receiver and the time offset of its local clock relative to GPS time. This local clock is typically based on an oscillator within the GPS receiver or attached to it, the local clock time being controlled by of the phase of the oscillator. Although at least four satellite ranging signals are needed for a three-dimensional position-plus-time solution, there are also a number of reversionary modes of operation that permit solutions to be obtained with 3 or 2 or, in some circumstances with only 1 satellite signal present. The present constellation of 30 operational GPS satellites is such that as many as 12 satellite signals may sometimes be received simultaneously, providing for reduced solution errors.
Similarly, the word ‘receiver’ when applied to the terminal's terrestrial transmission source receiver, should be understood in the same widest sense to include the radio receiver front-end, ancillary hardware, and one or more processors running software programs.
Although SPS receivers work very well in open-sky conditions, they are less successful in places where one or more of the satellite signals is subject to attenuation through blockage, for example inside a building. In these circumstances, the SPS receiver may benefit from support, both for signal acquisition and signal re-acquisition, from an alternative location technology which continues to operate. In addition, the location solutions, using SPS signals subject to blockage conditions, are often less accurate because of the radio propagation conditions, including reflections from nearby objects, refraction around edges, additional delays relative to the ideal ‘line-of-sight’ straight-line path between the receiver and the satellite, and interference between rays travelling on different paths.
Alternative methods for finding the position of a mobile terminal, adapted especially to cellular communications networks, use the communications signals themselves. These include the use of the position of the serving cell tower as a basis for a coarse position, measurements of the angles or times of arrival at base station sites of signals radiated by the terminal, and measurements of the times of arrival of signals radiated by surrounding base transceiver stations at the mobile terminal. In the last case, the time differences of arrival are usually measured, either with respect to the signals from the serving transmitter (the ‘serving cell’) or with respect to the terminal's local clock.
Cellular communication systems for mobile users may be classified into those systems in which the base station transmissions are synchronised to a common time standard and those whose base-stations are unsynchronised. Communications systems using the CDMA protocol usually fall into the synchronised category whilst most of the others, including wide-band CDMA (WCDMA) and GSM which together comprise more than 80 percent of the world-wide installed network base, are not synchronised. The CDMA system is synchronised with GPS time using GPS receivers installed at base-station sites.
A mobile terminal's position may be estimated from measurements of the times of reception of the signals from surrounding base transceiver stations. In practice, what is usually measured is the time of reception of an identified component in the modulation, for example, the synchronisation burst which is broadcast on the control channel in GSM, or the phase of the spreading code in a CDMA systems, or a frame boundary in any system. These measurements need to be combined with the positions of the respective transmitters, and knowledge of the transmission times or transmission time offsets of the identified components. In general, independent timing measurements of the signals from several geographically-disparate base station transceivers are needed, the minimum number depending on the number of unknown variable that must be estimated in the calculation. In the case of a two-dimensional position calculation using synchronised transmitters, three measurements are sufficient. In the case of a two-dimensional position calculation using unsynchronised (but stable) transmitters, measurements by two separated terminals (or one which moves between the two positions) of the signals from five transmitters are needed (see U.S. Pat. No. 6,529,165 and U.S. Pat. No. 6,937,866).
CDMA network transmissions are synchronised with GPS, and the transmission time offsets of the individual transmitters are broadcast. Measurements of the time of receipt (relative to GPS time) by the terminal of the signals from the base transceiver stations on different sites can then be converted to ranges from the terminal to each of those sites and the horizontal position of the terminal can be calculated unambiguously. Similarly, the time differences between two received signals provides the range difference between the two base station transmitter sites, and three such range differences is sufficient to provide for the unambiguous computation of the terminal's location.
GSM and W-CDMA networks are unsynchronised, and for these measurements of time differences are often employed. Of particular interest are positioning methods which use measurements made by the mobile terminal of the nearby base transceiver stations. The times of receipt of a selected component in the modulation of the received signals are measured with respect to one of them or a local clock in the terminal. In the so-called Enhanced Observed Time Difference method (E-OTD), the relative transmission time offsets are deduced from similar measurements made by one or more fixed monitoring receivers (Location Measurement Units; LMUs) at known positions. By combining the relative timings made by the terminal with the corresponding relative timings made by an LMU, the position of the terminal may be calculated using either hyperbolic (U.S. Pat. No. 6,275,705) or circular (EP 1,271,178) techniques.
A development of E-OTD, described in U.S. Pat. No. 6,529,165 and U.S. Pat. No. 6,937,866 avoids the use of LMUs by combining the measurements of many base transceiver stations made by many mobile terminals in a grand computation (hence the code-name ‘Matrix’) from which emerges the positions of all the mobiles together with the transmission time offsets of all the base stations. Minimum configurations include two terminals each measuring five base stations or three terminals measuring four base stations, but in practice many terminals and many base stations are used. The transmission time offsets of the base stations may also be used to compute and maintain a ‘Virtual’ LMU (VLMU) which is a list of the receive time offsets which would be measured by a single real LMU at a given location able to receive the signals from all the base stations (see WO 00/73813).
Note that in both the E-OTD method and its Matrix development a universal time of transmission or reception of the base-station signals is never determined. Only relative time differences are measured. This means that it is not possible to extract, for example, the GPS time from the receipt of a network signal by the terminal without making a calibration, either using a GPS-referenced LMU to align the VLMU list with GPS time, or by calibrating in the mobile terminal (see WO 2005/071430).
Each of the positioning methods outlined above can be used alone as the sole means of providing the location of mobile terminal. However, it is an advantage to combine two or more methods together, so that one can provide a position when the other fails, and the network-based method can assist the SPS method to acquire satellite signals. Of particular interest here is a combination of the Matrix method and an SPS method, say GPS. The Matrix method can provide a position fix within a second so that a user has a result almost instantly. However, the accuracy in GSM is about 100 m which may not be sufficient for the purpose. The cellular signals can then also be used to provide accurate time and position assistance to the GPS receiver so that it can obtain a position fix after only a few seconds even in difficult environments (see for example E-GPS: indoor mobile phone positioning on GSM and W-CDMA, Duffett-Smith, P. J. & Tarlow, B. Proceedings of the ION GNSS 2005 meeting, Long Beach, September 2005). What is needed is a way of combining these two systems together to provide the best possible positioning service, and this is addressed in the present application.